In this description we use the term gearwheel to refer to a cogged, rotating machine part. Two or more meshing gearwheels constitute a gear stage. The term gear as such refers in this description to a mechanical system having a first shaft and a second shaft, between which one or more gear stages provide speed and torque conversions and/or a change in a direction of a rotational axis. A gear unit comprises a gear proper and may comprise auxiliary augmenting systems, such as instrumentation, control, and lubrication arrangements.
The gear unit of a wind turbine plays an important part in transmitting the power and the rotation energy of the rotor blades to the generator, which creates electric energy. Ensuring the smooth and reliable operation of the gear is vital to achieving proper performance of the wind turbine. As such, gears are encountered also at numerous other applications, but the specific characteristics and demanding environmental conditions that concern a wind turbine mean that their design often calls for highly developed special solutions.
In a wind turbine the first shaft of the gear is coupled to the rotor assembly and the second shaft is coupled to the generator. Within the gear, at least one gear stage is located between the first and second shafts for transmitting the rotational movement and for implementing the desired speed and torque ratios between the shafts. Bearings are used to support the shafts and the gearwheels with as little friction as possible.
The gear stage(s) and the bearings need lubrication, for which purpose the gear unit typically comprises a lubrication pump configured to circulate lubrication fluid through the gear stages and bearings. In very simple implementations a rotational movement of some part of the gear is used to drive the lubrication pump. Concerning a wind turbine, which may stand still for lengthy periods, such an axis driven lubrication pump would cause a delay at startup before a sufficient amount of lubrication fluid could reach the critical moving parts, and is thus not to be recommended.
A more versatile solution involves using a dedicated power source, such as an electric or hydraulic motor, to drive the lubrication pump. As a part of a controlled startup the power source may be switched on before the gearwheels start moving, so that they receive the appropriate amount of lubrication fluid already from the beginning. Knowing when to switch on the power source obviously necessitates the gear unit to comprise, or to benefit from the operation of, a control unit that is capable of controlling the operation of large parts of the wind turbine.
However, even in such cases finding the proper lubrication method is not straightforward. As an example, temperatures ranging from −40 to over +85 degrees centigrade may occur within the gear, with significant effects on the viscosity of the lubrication fluid. Changes in viscosity affect the way in which the lubrication fluid flows through the lubrication channels, and also its capability of maintaining the lubricating film between metal parts moving in relation to each other.